1. Field of the Invention
The present invention relates to an image processing method for correcting color image data. More specifically the invention pertains to a technique of quick image processing by referring to a correction table.
2. Description of the Related Art
A known image processing apparatus reads a color original with an image input unit, such as a scanner, to input color image data and reproduces the input color image data with an image output unit, such as a CRT, another display, or a color printer.
Each of the image output units like displays and color printers has intrinsic color reproduction properties. Color correction according to the color reproduction properties of each image output unit is accordingly required for favorable reproduction of the colors of a color image input with a scanner or another image input unit, regardless of the properties of the image output unit. A typical technique of color correction uses a color correction table (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 4-185075). The color correction table is a numerical table that stores a mapping of corrected color image data to target color image data, which is the object to be corrected. For example, in the case of target color image data expressible by three color components, red (hereafter referred to as R), green (hereafter referred to as G), and blue (hereafter referred to as B), the technique divides a color space defined by three orthogonal axes of these color components into lattices and creates a three-dimensional numerical table that stores corrected color image data at each of lattice points. The procedure detects a coordinate point corresponding to target color image data and peripheral lattice points in the vicinity of the coordinate point in the color space and carries out interpolation of the corrected color image data stored at the peripheral lattice points, so as to specify the corrected color image data corresponding to the target color image data.
The applicant of the present invention has developed and filed a technique of quick color correction by referring to a color correction table without such interpolation (disclosed by JAPANESE PATENT LAID-OPEN GAZETTE No. 9-294212). This technique forcibly allocates the coordinate point corresponding to target color image data to a peripheral lattice point in the vicinity of the coordinate point. The color conversion table stores corrected image data at each lattice point. Allocation of the coordinate point of the target color image data to the peripheral lattice point ensures quick conversion of the target color image data into corrected color image data without interpolation. In the description hereafter, the process of forcibly allocating the coordinate point corresponding to target color image data to the peripheral lattice point prior to referring to the color correction table is called the ‘prior art pre-conversion process’. The corrected color image data obtained from the lattice points allocated by the prior art pre-conversion process is different from the original corrected color image data obtained by direct color correction of the target color image data. There is an error in individual image data. In the description hereafter, the error arising due to pre-conversion is called the ‘pre-conversion error’. The preferable procedure of pre-conversion enables the pre-conversion errors arising in the individual color image data to be mutually cancelled and makes the total error in a predetermined image area within an allowable level. This desirably prevents the image quality from being worsened by the pre-conversion error.
In the case of extremely large pre-conversion errors arising in individual image data, however, even if the mutual cancellation of the pre-conversion errors causes the total pre-conversion error in a predetermined image area to be not greater than an allowable level, the individual pre-conversion errors may be highly noticeable to worsen the picture quality. In such cases, the effective measure is to make lattice point intervals sufficiently dense in the color correction table. The arrangement of making the dense lattice point intervals and setting a large number of lattice points enables some of the lattice points to be present close to the coordinate point of the target color image data and thereby reduces the pre-conversion error. In other words, it is preferable to use the color correction table having a setting of lattice points at adequate intervals, in order to prevent the occurrence of the pre-conversion error exceeding the allowable level.
With a recent improvement in display quality, the allowable level of the pre-conversion error becomes stricter. The proposed method to attain the lowered pre-conversion error uses the color correction table having a dense setting of lattice point intervals. The denser setting of lattice point intervals, however, naturally raises the total number of lattice points and thereby increases the amount of data stored in one color correction table. This leads to an increase in storage capacity required for the color correction table. The increased amount of data in the color correction table lowers the processing speed of color correction and prevents quick output of a resulting image.
The interpolation technique can reduce the pre-conversion error without the dense setting of lattice point intervals. The interpolation, however, undesirably extends the total processing time and thereby prevents the high-speed color conversion.